This invention relates in general to capillary devices and in particular to rectangular capillaries useful in capillary electrophoresis (CE), particularly in capillary zone electrophoresis (CZE).
Capillary electrophoresis is one of the most powerful separation techniques for the analysis of a wide variety of complex mixtures. The technique is capable of orders of magnitude higher resolution than high-performance liquid chromatography; moreover, with CE work it is possible to analyze nanoliter samples. In the past, separation in CE has been exclusively performed in circular capillaries with internal diameters between 5 and 200 microns. The small size of the capillary allows extremely efficient heat dissipation, but as the capillary dimensions are increased beyond 100 microns, a dramatic decrease in separation efficiency is observed. Consequently, CE cannot be scaled to larger diameter capillaries, even with efficient cooling of the outside of the capillary by heat transfer fluids. Furthermore, with circular capillaries, CE cannot be used for ultra-low concentration applications. That is, while the mass sensitivity of CE is outstanding, detection methods still remain the "Achilles heel" of the technique. The ability to detect low concentrations in a 100 micron capillary is difficult, especially when using the very common technique of UV-Vis absorbance.
Another inherent problem associated with conventional circular capillaries is the optical distortion caused by the curvature of the capillary walls. This problem is particularly important when optical detection means are utilized. For example, the curvature at the solute (liquid)-wall interface or at the wall-atmosphere (detector) interface will adversely affect refractive index or photodeflection measurements. In addition, when direct counting methods are employed, the curvature of the capillary walls can cause inaccurate counts.
CZE in small capillaries has proven useful as an efficient method for the separation of solutes. An electric field is applied between the two ends of a capillary tube into which an electrolyte containing the solutes is introduced. The electric field causes the electrolyte to flow through the tube. Some solutes will have higher electrokinetic mobilities than other solutes so that the solutes form zones in the capillary tubes during the flow of the electrolytes through the capillary. However, Joule heating owing to the ionic current carried between the electrodes can result in temperature gradients and subsequent convection and density gradients that increase zone broadening, affect electrophoretic mobilities and even lead to boiling of solvent.
There is a critical need for a capillary device that handles large throughputs and dissipates heat efficiently in CE. Moreover, there is a need for capillary devices with sufficient cell pathlengths so that detection of low concentration samples are facilitated. Furthermore, the capillary device should create minimal optical distortions. Finally, conventional circular capillaries are not suitable for two-dimensional CE separation. A need exists for capllary devices that offer this option.